Currently, portable media devices can connect to host units and can transmit content to the host units as well as exchange content between other devices connected to the host units. Current technology also allows the host unit to control the playback of content from the portable media device. For example, a portable audio player can connect to an automobile's media receiver and the portable audio device's content can be controlled and played back from controllers on the automobile media receiver or through controllers integrated within the automobile's steering wheel.
Once a portable media device is connected to a host unit, the host unit can control playback of content on the portable media device by first reading the particular device and then reading the content stored on the portable device. The host unit can then receive the content from the portable device and playback the content using its controllers. Such an implementation is literally a remote transfer followed by playback; as this implementation does not actually play the content from the portable device. This implementation is disfavored as the content is transferred away from the portable media device to the host unit and then played from the host unit.
A problem with this implementation arises when dealing with portable media devices that contain Digital Rights Management (DRM) protected content. With DRM protected content, the portable media devices themselves must be registered in order to subscribe to receive the DRM protected content. Only registered devices can receive and playback DRM protected content. One's automobile media receiver, set-top box, or printer could not have the DRM protected content transferred to them as they would not be legitimate devices registered to receive and playback the DRM protected content. Another problem is that a host unit may be able to playback one particular metadata or codec such as MP3, however, not all host units can playback all different types of metadata and codecs. So a situation could arise where a registered portable media device may be able to play the DRM protected content, however, various host units may not be able to playback the different formats of content.
Another way in which current technology attempts to allow a host unit to control playback of a portable media device is to enable the host unit to send interrupt-based commands to the portable media device: such as telling it to skip, play, pause, etc. The problem with having an interrupt-based system of controlling portable media devices is that it does not allow the host unit to have an existing playback paradigm. By controlling through interrupts, the host unit will not know what state the device is in, which will lead to host units trying to remote the user interface of the portable media device to the controller of the host unit, instead of allowing the controller to verify what is on the device and to check what is the playback state of the device.